LDPE has been produced in autoclave reactors, tubular reactors, and combinations thereof. Each type of reactor has its advantages and disadvantages, but economics and product design drive the need for improvements. The operation of and type(s) of reactor(s) employed can dramatically affect the physical properties of the resulting LDPE. Such improvements are desired for applications such as blown and cast film, where especially good optical properties are desired.
High pressure, low density ethylene-based polymers have a density in a range of about 0.91 to about 0.94 g/cm3. Low density ethylene-based polymers typically have random branching structures that contain both alkyl substituents (short chain branches) as well as long chain branches. Most LDPE polymers are homopolymers, although some are copolymers and interpolymers, typically using other α-olefin comonomers.
Chain transfer agents (CTAs), or “telogens”, are often used to control the melt index in a free-radical polymerization process. “Chain transfer” involves the termination of growing polymer chains, thus limiting the ultimate molecular weight of the polymer material. Chain transfer agents are typically hydrogen atom donors that react with a growing polymer chain and stop the polymerization reaction of the chain. Known CTAs include many types of hydrogen atom donor compounds, such as saturated or unsaturated hydrocarbons, aldehydes, ketones, and alcohols. By manipulating the concentration and type of chain transfer agent used in a process, one can affect the average length and molecular weight distribution of the polymer chains. This in turn affects the melt index (I2 or MI), which is related to molecular weight.
Many chain transfer agents are known in the art for use in high-pressure, low density polyethylene production. References that disclose the use of chain transfer agents in free-radical polymerization of ethylene and ethylene-based polymers include Ehrlich, P., and Mortimer, G. A., “Fundamentals of the Free-Radical Polymerization of Ethylene”, Advanced Polymers, Vol. 7, 386-448 (1970); Mortimer, George A., “Chain Transfer in Ethylene Polymerization—IV. Additional Study at 1360 Atm and 130° C.”, Journal of Polymer Science, Part A-1, Vol. 8, 1513-23 (1970); Mortimer, George A., “Chain Transfer in Ethylene Polymerization—VI. The Effect of Pressure”, Journal of Polymer Science, Part A-1, Vol. 8, 1543-48 (1970); Mortimer, George A., “Chain Transfer in Ethylene Polymerization—VII. Very Reactive and Depletable Transfer Agents”, Journal of Polymer Science, Part A-1, Vol. 10, 163-168 (1972); Great Britain Patent No. 997,408 (Cave); U.S. Pat. No. 3,377,330 (Mortimer); U.S. Patent Publication No. 2004/0054097 (Maehling, et al.); and U.S. Pat. Nos. 6,596,241; 6,673,878; and 6,899,852 (Donck).
After hydrogen atom donation, it is known that a chain transfer agent may form a radical which can start a new polymer chain. The result is that the original CTA is incorporated into a new or existing polymer chain, thereby introducing a new functionality into the polymer chain associated with the original CTA. The CTA may introduce new functionality into the polymer chain that is not normally the result of the monomer/comonomer polymerization.
Low density ethylene-based polymers produced in the presence of CTAs are modified in a number of physical properties, such as processability; film optical properties such as haze, gloss and clarity; density; stiffness; yield point; film draw; and tear strength. For example, an α-olefin acting as a CTA could also introduce a short chain branch into a polymer chain upon incorporation.